1. Field of the Invention
The present invention relates to a coil antenna preferably to be provided in a portable electronic apparatus and a portable electronic apparatus including the coil antenna.
2. Description of the Related Art
RFID (Radio Frequency Identification: non-contact data identification technology, radio-wave system recognition) systems using non-contact IC cards or IC tags have been introduced in various fields, such as automatic train-ticket gates, security systems for entering and leaving buildings, and electronic money systems, for example.
Such an RFID system includes a non-contact IC card, which includes an IC chip and a planar coil to be used for data transmission/reception and data storing, and a reader/writer for writing and reading data to and from the non-contact IC card. Magnetic flux from the reader/writer passes through the planar coil, so that communication between the non-contact IC card and the reader/writer can be performed.
In recent years, a technology for communication between a cellular phone including an IC chip, instead of a non-contact IC card in the above-described RFID system, and a reader/writer have become commonplace.
In a cellular phone including such an IC chip, a coil antenna having a large loop area or a coil antenna formed by winding a coil around a magnetic core is disposed in a casing of the cellular phone. Thus, such a cellular phone is disposed close to an antenna of a reader/writer so that magnetic flux from the antenna of the reader/writer can pass through the coil of the coil antenna.
In a cellular phone or other suitable electronic component including such a non-contact IC card chip, the planar coil is provided on one of the surfaces of the casing of the cellular phone. However, since a circuit board including a ground electrode, a metallic battery case, and other components are provided inside the cellular phone, it is difficult to cause magnetic flux to pass through the planar coil. Thus, there is a problem of not achieving a wide communication range. In addition, for example, in a case in which the cellular phone is disposed close to the reader/writer such that the front and back sides of the cellular phone are inverted from an intended orientation, communication may not be performed at all. Accordingly, there is a problem of variations in sensitivity, depending on the orientation of the cellular phone.
A non-contact IC card (an RFID medium) which efficiently receives necessary electric power is disclosed in Japanese Unexamined Patent Application Publication No. 2000-201442. The non-contact IC card described in Japanese Unexamined Patent Application Publication No. 2000-201442 includes a communication antenna, which transmits and receives data and signals without contact, and a plurality of receiving coils. In a state in which the receiving coils face feeding coils with gaps therebetween, electromagnetic induction is generated between the receiving coils and the feeding coils. In the case of receiving power, the receiving coils receive a power supply from the feeding coils without contact by the electromagnetic induction.
In addition, an RFID medium which provides communication between a reader/writer and the RFID medium even when the RFID medium is disposed in a direction that is perpendicular to a coil antenna of the reader/writer is described in Japanese Unexamined Patent Application Publication No. 2004-126750.
The configuration of a non-contact IC tag disclosed in Japanese Unexamined Patent Application Publication No. 2004-126750 is shown in FIG. 1. In the non-contact IC tag, an IC module 210 to and from which information is externally written and read and a conductive antenna unit 220 for supplying electric power to the IC module 210 using electromagnetic induction from a reader/writer and for performing writing and reading of information to and from the IC module 210 are provided on a base member 250, which is a resin sheet, for example. The antenna unit 220 is connected to the IC module 210 such that two coil antennas 221a and 221b are connected in series so as to have a figure eight shape.
However, for such a known RFID medium, when communication is performed to authenticate a user, billing and settlement, or other suitable purposes by moving a portable electronic apparatus close to a reader/writer, the communication performance (communication range) greatly changes, depending on the location of a communication antenna provided in the portable electronic apparatus, that is, depending on whether the communication antenna is close to a central portion of the reader/writer or close to a location spaced from the central portion (i.e., a location displaced from the central portion) of the reader/writer. This change causes a problem of a reduction in the communication range where an RFID medium is close to a peripheral portion of a reader/writer.
With the non-contact IC card described in Japanese Unexamined Patent Application Publication No. 2000-201442, providing a plurality of receiving coils in the RFID medium increases the power reception efficiency. However, since only a single communication antenna coil is provided in the non-contact IC card, a problem of deterioration in the communication performance in a case where the RFID medium is brought closer to a reader/writer such that the position of the RFID medium is deviated from the reader/writer cannot be solved.
In addition, Japanese Unexamined Patent Application Publication No. 2004-126750 is designed in order to provide a predetermined communication range even when the positional relationship between an RFID medium and a reader/writer is changed from a known direction in which the RFID medium and the reader/writer face each other to a direction in which the RFID medium and the reader/writer are substantially perpendicular to each other. Thus, Japanese Unexamined Patent Application Publication No. 2004-126750 still has a problem of deterioration in the communication performance when the RFID medium is moved close to the reader/writer such that the location of the RFID medium is deviated from the reader/writer. Furthermore, since a plurality of antenna coils are connected in series such that the winding directions of the antenna coils are opposite to each other, when magnetic fluxes pass through both coils, induction currents cancel each other, resulting in a relatively narrow communication range.